Why doesn't the 'Messenger Probe' Gravity Assist work in SFS to save fuel to Mercury?

[Joshsstuff]

In the Messenger Probe mission, several gravity assists were used to slow the spacecraft down, thus saving fuel on the final insertion burn to Mercury.
This is very important, because Mercury has no atmosphere, thus no Aerobraking is possible.


This "Messenger" mission was from Earth to Mercury.
And 3 consecutive encounters with Mercury was able to shrink the orbit & speed for a more economical Mercury Injection burn.

My mission (Shown below) was from Venus to Mercury.

While I was successful in using multiple encounters with Mercury to shrink the Appilapsis,
the resulting injection burn was not more efficient than a direct transfer from Venus.

It would seem that this smaller altitude orbit should result in fuel savings to orbit Mercury, but the more aggressive I was to get Gravity Assists, the more fuel it took in comparison to a standard Venus to Mercury transfer.

I was able to do better by ONLY using the free encounters and not spending any fuel to create them. (This took a long time)
But I've still not seen any advantage like the Messenger Probe Mission did.

Is this a problem with SFS's physics?
To be clear, the Gravity Assists were working as you would expect to raise or lower my orbit.
In fact, If I wanted to, I could use this approach to go anywhere in the SFS solar system and Aero-brake on Earth or Mercury for huge fuel savings.

The problem is that it seems to take the same amount of fuel to circularize orbits for injection, no matter the Apoapsis.

This is perplexing because the original Messenger Mission even used Deep Space Maneuvers, using fuel to create it's multiple Mercury encounters, and was able to save a huge amount of fuel doing it.

Has anyone had more success braking your speed with gravity assists in the game?

 

[GuHP20]

In the Messenger Probe mission, several gravity assists were used to slow the spacecraft down, thus saving fuel on the final insertion burn to Mercury.
This is very important, because Mercury has no atmosphere, thus no Aerobraking is possible.

View attachment 3536
This mission was from Earth to Mercury.
And 3 consecutive encounters with Mercury was able to shrink the orbit & speed for a more economical Mercury Injection burn.

My mission was from Venus to Mercury.
While I was successful in using multiple encounters with Mercury to shrink the Appilapsis,
the resulting injection burn was not more efficient than a direct transfer from Venus.

View attachment 3571

View attachment 3572


It would seem that this smaller altitude orbit should result in fuel savings to orbit Mercury, but the more aggressive I was to get Gravity Assists, the more fuel it took in comparison to a standard Venus to Mercury transfer.

View attachment 3570

I was able to do better by ONLY using the free encounters and not spending any fuel to create them. (This took a long time)
But I've still not seen any advantage like the Messenger Probe Mission did.

Is this a problem with SFS's physics?
To be clear, the Gravity Assists were working as you would expect to raise or lower my orbit.
In fact, If I wanted to, I could use this approach to go anywhere in the SFS solar system and Aero-brake on Earth or Mercury for huge fuel savings.

The problem is that it seems to take the same amount of fuel to circularize orbits for injection, no matter the Apoapsis.
View attachment 3536

This is perplexing because the original Messenger Mission even used Deep Space Maneuvers, using fuel to create it's multiple Mercury encounters, and was able to save a huge amount of fuel doing it.

Has anyone had more success braking your speed with gravity assists in the game?

This is...complex...

 

[Joshsstuff]

This is...complex...

The maneuvers are complex, however the principle is simple.

Just as gravity assists can save you fuel by speeding up your rocket and raising it to a higher orbit,
you can do the reverse to lower/slow down.

This is useful in real life as was used in the Nasa probe to Mercury because it takes a lot of fuel to slow down once you reach Mercury unless you let encounters with planets help you brake.

I wish to use this for the R.O.L. Challenge to save fuel, but so far it's not working.
SFS Forum R.O.L Challenge (Take a Rover to Every Planet!)

Reddit R.O.L. Challenge Link

 

[Blazer Ayanami]

Ahhh.....
Why don't you ask @Altaïr ? He is the teacher of Gravity Assistance.

 

[Altaïr]

Wow, interesting, I never thought to use a gravity assist from Mercury like this.

First of all, you should be careful when comparing this game with reality. Everything is scaled down and is much much easier in this game (you surely noticed you didn't need a Saturn to go the Moon way and back for example). The gravity assist provides a comparable advantage, but if you don't use it, you'll need much less fuel than in reality. Without gravity assist, going to Mercury in this game "just" uses more fuel, in reality that's impossible. So this maneuver is much more valuable in reality than in the game.

But it can still be useful. Concerning Mercury, it's a small planet, so it doesn't provide a big shot, which is why you have to do it several times to get a significative advantage from it. So if you burn fuel like crazy into maneuvers just to get an encounter, you will quickly negate the advantage of the slingshot. It's better to wait a good opportunity, so that you have a minimal correction to perform. That's basically what space agencies do in reality. Actually, saving fuel with that maneuver comes at the cost of a huge extra mission time.

A word concerning the comparison with a direct injection from Venus: in this case, you use (maybe without being aware of it) another powerful maneuver, which is the Oberth effect. It consists in burning prograde while you are close to a massive body (Venus here), which provides you a bigger benefit than if you burned far from that body (space mechanics are really amazing aren't they? ). When you perform the slingshot, you partly lose that effect. Actually, you compare two powerful maneuvers, which is why the benefit doesn't look so great in comparison.

Oh, something to be aware of too (actually, I realized this myself recently ), is that the extra speed you get from a slingshot doesn't depend on the mass of the object you want to accelerate. This means that you'll get the same extra speed if you do that maneuver with a small probe, or with your bigger space station. What makes a difference is that you would need to use a LOT of fuel to get the same result in the case of the space station, whereas you would just need a bit of fuel for the probe. So that maneuver is actually more valuable when it concerns heavy objects.

Wow, this message gets really long! Tell me if that answers your question, and don't hesitate if you still have something to ask

 

[Joshsstuff]

The gravity assist provides a comparable advantage, but if you don't use it, you'll need much less fuel than in reality. Without gravity assist, going to Mercury in this game "just" uses more fuel, in reality that's impossible. So this maneuver is much more valuable in reality than in the game.

But it can still be useful. Concerning Mercury, it's a small planet, so it doesn't provide a big shot, which is why you have to do it several times to get a significative advantage from it. So if you burn fuel like crazy into maneuvers just to get an encounter, you will quickly negate the advantage of the slingshot. It's better to wait a good opportunity, so that you have a minimal correction to perform. That's basically what space agencies do in reality. Actually, saving fuel with that maneuver comes at the cost of a huge extra mission time.

Yeah, I'm sure the real space mission could have saved even more fuel, but the mission would take 100 years
But since time is unlimited, but fuel is not for the Rover challenge, I'm looking for the most fuel efficient way.
On my latest attempts I waited a long time so that I used free encounters, and It was closer to the 'Direct transfer', but no savings of fuel.

A word concerning the comparison with a direct injection from Venus: in this case, you use (maybe without being aware of it) another powerful maneuver, which is the Oberth effect. It consists in burning prograde while you are close to a massive body (Venus here), which provides you a bigger benefit than if you burned far from that body (space mechanics are really amazing aren't they? ). When you perform the slingshot, you partly lose that effect. Actually, you compare two powerful maneuvers, which is why the benefit doesn't look so great in comparison.

This is a good point about the Oberth effect.
If I used multiple venus encounters to get to my first Mercury encounter, then I'm trading the advantage of the Oberth effect.
However, if my 1st encounter with Mercury comes directly from Venus, then I'm getting the same transfer as a "direct" transfer, the only difference is that I fly-by for a gravity assist vs. injection burn for the direct transfer.

Oh, something to be aware of too (actually, I realized this myself recently ), is that the extra speed you get from a slingshot doesn't depend on the mass of the object you want to accelerate. This means that you'll get the same extra speed if you do that maneuver with a small probe, or with your bigger space station. What makes a difference is that you would need to use a LOT of fuel to get the same result in the case of the space station, whereas you would just need a bit of fuel for the probe. So that maneuver is actually more valuable when it concerns heavy objects.

Wow, this message gets really long! Tell me if that answers your question, and don't hesitate if you still have something to ask

This point is relevant to the Mercury gravity assist as well.
The only point of using gravity assist was to slow down my speed by lowering my Apoapsis.
(The mass of your rocket does make a difference for the injection burn, and I wished to lower the speed since F=MA equation)
I was successful in lowering my apoapsis without using much fuel at all (2nd picture of O.P.) but this does not seem to help when I do my final injection burn.

speaking of the Oberth effect, does this work in reverse?
Or, I should ask, "does SFS care if I make my orbital insertion burn as close to Periapsis of the destination planet as possible"?
I will try and see if I can make any difference.

I have been reading up on Bielliptic transfers, that are more efficient than Hohmann when the semi-major axis is >12.
(the Soyuz uses this to rendezvous with the ISS)
I must admit that I barely have a grasp of the concept. Are you familliar with it?
Is it helpful in SFS? (it does take advantage of the Oberth effect much more than hohmann transfers)

Thank you for any help you can offer!

 

[Altaïr]

Oh, you're right for the Oberth effect, as you start from Venus that doesn't make a difference. I'll give it a try to see by myself I think.

The Oberth effect also works when you slow down indeed, so it's better to make your injection burn at the periapsis. I've noticed a great difference recently when making the Grand Tour: I first wanted to satellize a refuel ship in high orbit around Mercury, because aiming for a low orbit would have required to spend more energy. Seems logical? Because of the Oberth effect that was wrong! Satellizing in low orbit was actually far more efficient.

I've heard about the bi-elliptic transfer, but I never tried it, so I can't help much for this. I'm not convinced the gain would be huge though... There is still one situation in which I make an intermediate transfer, it's when I want to return on Earth from Deimos: I quit Deimos SOI, and first lowers my orbit around Mars. Thus, when I burn to get my ship onto a transfer orbit, I benefit from the Oberth effect from Mars.
But the gain is not huge. Well, that's also because my ship is generally very light, and like for the gravitational slingshot, the gain is limited in this case. The main reason why I do this is because it's easier to manage, due to the transfer orbit being hard to be matched when you're on a high orbit.

 

[Altaïr]

Ok, I gave it a try. I sent a probe to Venus for a gravity assistance, and from there, I tested different scenarios.
Here is my probe, released during the Venus fly-by and its trajectory:

As you can see that's a very small ship, so I won't have a lot of margin
In all cases, I tried to satellize at around 10 km, so that there is no difference from the Oberth effect due to a burn at a different altitude.
So, here is the result with a direct injection:

And here after several maneuvers to reduce the orbit:

The result is... practically the same
So now I understand better your problem.
But I think I understood what was the problem. In this case, I could lower the apoapsis, which is good, but I also reduced the periapsis, which is bad! Both parameters count actually, they must be as close as possible to Mercury orbit to minimize the fuel needed for the injection burn.
So I tested another strategy (which seems to be what Messenger did): after each slingshot, I burnt at the apoapsis to raise the periapsis at Mercury level:

I finally inserted in this situation:

If you look at the fuel gauge, you can see that I finally managed to save fuel, despite the maneuvers I had to do. So I think that's how it works.

Now the question is: is it worth it? After all, I saved something like 0.5 tons of fuel. That's for a very small probe, so that's relatively good, but it's really easy to add an additional 2.5t fuel tank if needed.
The benefit may be more significatice with a large ship though

 

[Joshsstuff]

Oh, you're right for the Oberth effect, as you start from Venus that doesn't make a difference. I'll give it a try to see by myself I think.

The Oberth effect also works when you slow down indeed, so it's better to make your injection burn at the periapsis. I've noticed a great difference recently when making the Grand Tour: I first wanted to satellize a refuel ship in high orbit around Mercury, because aiming for a low orbit would have required to spend more energy. Seems logical? Because of the Oberth effect that was wrong! Satellizing in low orbit was actually far more efficient.

Is the re-Fueling for the Grand Tour challenge?
I want to try this, but I'm focused on finishing the Rover Challenge 1st.

It seems that the Oberth effect is the most pronounced in SFS.
I thought this might make a big difference, as I understand the Oberth effect, it reminds me of mechanical advantage as in a machine that uses levers or gears. (being close to a planet has a greater effect, like a 'high gear' vs. being away from the planet you must use high RPMs for the same effect)
The timing to Oberth an insertion burn will be challenging for me because I'm using ION engines so I'll need to plan the exact starting point of the burn much like a "hover slam"/ "Suicide burn"

I've heard about the bi-elliptic transfer, but I never tried it, so I can't help much for this. I'm not convinced the gain would be huge though... There is still one situation in which I make an intermediate transfer, it's when I want to return on Earth from Deimos: I quit Deimos SOI, and first lowers my orbit around Mars. Thus, when I burn to get my ship onto a transfer orbit, I benefit from the Oberth effect from Mars.
But the gain is not huge. Well, that's also because my ship is generally very light, and like for the gravitational slingshot, the gain is limited in this case. The main reason why I do this is because it's easier to manage, due to the transfer orbit being hard to be matched when you're on a high orbit.

Mars' moons can be quite awkward to deal with because of their low gravity.
So, from Deimos, you burn past it's tiny SOI then just fall toward Mars? (do I understand you correctly?)

With all of the problems I've had using gravity assist to slow down to Mercury, It is super useful to go the opposite way!
For the ROL Challenge my path is to go from Mercury to Earth.
Especially since the long burn directly to Earth is to large for the transfer window calculated by SFS (for a ION engines) It's a mess trying to guess how to get there directly.

Using 2 Oberths @ Mercury and an assist @ Venus however, I use 1/2 the fuel or better!
(it's like getting to Earth + for the price of Venus! & I can use much more precise transfers)

 

[Altaïr]

Yes, the refueling was for the Grand Tour challenge. I described it here:
https://jmnet.one/sfs/forum/index.php?threads/grand-solar-system-tour-challenge.826/#post-9533

I agree, the Oberth maneuver is powerful and yet simple to achieve. The gravitational slingshot will generally give better results, but at the cost of complexity, as this is a tricky maneuver. I used it a lot for the Grand Tour, and that allowed a lot of optimization, which alleviated considerably my ship.

 

[Joshsstuff]

Blazer was right, your the right guy to ask, You saved quite a bit of fuel!
I am trying to figure out exactly how you did it, and the function of the "Deep Space Maneuvers"
I originally thought that the maneuvers purpose was to get a faster encounter with Mercury, but you are pointing to another purpose.

But I think I understood what was the problem. In this case, I could lower the apoapsis, which is good, but I also reduced the periapsis, which is bad! Both parameters count actually, they must be as close as possible to Mercury orbit to minimize the fuel needed for the injection burn.
So I tested another strategy (which seems to be what Messenger did): after each slingshot, I burnt at the apoapsis to raise the periapsis at Mercury level:
View attachment 3605 View attachment 3606
I finally inserted in this situation:
View attachment 3607 View attachment 3608 View attachment 3609

It looks like your goal is to synchronize the orbit with Mercury (the same method that is used to dock with any object)
Is this the goal in saving injection speed?
If so, I can understand it, because this is the point at which the relative velocity between the ship/target would be lowest. Am I right?

I burnt at the apoapsis to raise the periapsis at Mercury

Can you explain how the periapsis factors into this? (unless the purpose is to lower the relative speeds as I said above)
My original idea was to lower the periapsis and encounter mercury at the apoapsis (because the orbital speed is slowest at apoapsis)
Am I wrong to look at the orbital velocity and just focus on the relative velocity being the same?

If you look at the fuel gauge, you can see that I finally managed to save fuel, despite the maneuvers I had to do. So I think that's how it works.

Now the question is: is it worth it? After all, I saved something like 0.5 tons of fuel. That's for a very small probe, so that's relatively good, but it's really easy to add an additional 2.5t fuel tank if needed.
The benefit may be more significatice with a large ship though

That is significant! Good job

 

[Altaïr]

Yes, the deep space maneuvers are done to adjust the trajectory, but what I did was burning to adjust the periapsis so that it doesn't get too low compared to Mercury orbit. Maybe they wanted to do this in the real mission though, but I really can't say. Moreover, the problem is more complex in reality, because Mercury orbit is elliptic, and its orbit is not in the same plan than Earth's orbit. I don't know how to take this into account in the real situation. All I can do is trying and see the result

You're correct about the periapsis and apoapsis. Having them as close as possible to your target's orbit will mean a smaller relative velocity difference, and thus a smaller injection burn. In any case you will still have to absorb the velocity gained from Mercury's gravity though.
Lowering the periapsis and meet Mercury at the apoapsis is not a good idea because of this. Your speed will be minimal in this situation, that's correct, but Mercury will be faster than you then. So the relative speed will be still important.
I'll continue to experiment (even if new stuff from the next update should be my priority ), I'll tell you if I find something interesting

 

[Blazer Ayanami]

I took screenshots of all replies of you two to read them at home more carefully.

 

[Altaïr]

Oh cool, I did a new try, and managed to save even more fuel:

Here is the screenshot from the direct burn injection for a comparison:

And what's funny is that I exactly needed 3 slingshot maneuvers like the real probe, so that strategy seems to be the good one.
So the trick consists in adjusting your periapsis at Mercury level. After each slingshot, your apoapsis is lowered, but so is the periapsis. So you have to burn prograde at the apoapsis to raise again the periapsis at Mercury level. When the 2 orbits are really close to each other, you can insert into Mercury orbit, and it's over.

 

[Joshsstuff]

Oh cool, I did a new try, and managed to save even more fuel:
View attachment 3712
Here is the screenshot from the direct burn injection for a comparison:
View attachment 3713
And what's funny is that I exactly needed 3 slingshot maneuvers like the real probe, so that strategy seems to be the good one.
So the trick consists in adjusting your periapsis at Mercury level. After each slingshot, your apoapsis is lowered, but so is the periapsis. So you have to burn prograde at the apoapsis to raise again the periapsis at Mercury level. When the 2 orbits are really close to each other, you can insert into Mercury orbit, and it's over.

Ahh, that makes sense.
It's tough because it's so counterintuitive on the retro side as compared to pro-grade assists.
It really does take experimentation to find out how things work.

Thank you for helping us figure this out!
You were the right man to ask for this task